Massive units like process reactor vessels, furnaces, process steam and power production boilers, turbines, and other devices benefit from pre-heating to prevent damage by heating up too fast or other damage caused by low temperature startup. One example is a typical Fluidized Bed Catalytic Converter found in numerous refineries. These units can be heated up with steam, however at temperatures below 100° C. the steam can condense. The condensate can then be absorbed by the significant amounts of refractory. The steaming can heat up the unit very quickly, however if done too quickly the condensate absorbed into the refractory can flash off very quickly causing significant damage. Given the costs associated with downtime with systems like this, a need exists to quickly heat up these units in a controlled manner after maintenance cycles or other outages.
In the power industry, electricity is produced with a spinning turbine that is turned at high speeds to generate electricity. This turbine can be turned by water, by gas, or by high temperature steam. A steam turbine is driven by high temperature steam from a conventional boiler or nuclear reactor at speeds averaging 1800 to 3600 rpm. Many of the modern stream turbines operate at temperature in excess of 500° C.
Units such as these turbines experience substantial heat up problems associated with planned major outages, planned minor outages, and unplanned outages. After the mechanical repairs and replacement parts are installed on a cold turbine, the turbine needs to be readied for use. In most cases the only option to heat up the turbine is to introduce a full flow of steam into the turbine resulting in very aggressive heating which can damage the equipment. There are two primary issues on steam turbine heat ups. Various seals that control and direct the steam flow through the turbine do not properly seat and properly direct that flow until these metal seals are heated and expanded with temperature. Using uncontrollable low temperature steam that is saturated with moisture that is not following the designed flow paths due to the turbine seals not initially seating properly, causes damage in the form of erosion corrosion on turbine parts. Second issue, when these new parts are installed, the parts do not exactly fit the wear area of the old part that was replaced. As the cold turbine is placed on line, vibrations form that can be excessive with these new parts not seating properly. This requires shutting the unit back down and mechanically adjusting the new parts and rebalancing the machine and then trying to start the machine back up. Even and controlled heating of the turbine prior to startup, alleviates most of these vibration problems by preheating the new parts to expand and properly seat to a position intended by the turbine design, saving several days in the startup of a refurbished turbine. The operator of the machine should, over a period of several hours, carefully preheat (prewash) the turbine at a slow rate, prior to placing the turbine into production. Steam prewashing can only be controlled by the rate of the steam injection, since temperature control of the steam is not readily obtainable. This method gives a controlled rate of heating by using a nitrogen prewash at controlled temperatures and flows. Alternatives to this aggressive heating are to use a stable vapor to heat the turbine up in a controlled manner to a safe temperature before opening the steam control valves. It is envisioned that augmented heat up of a steam turbine may result in some start-up timesavings of about 4 to about 40 hours to heat up the system back to an operational level. This inefficiency represents a substantial amount of lost production and associated revenues for a given generating unit on an annual basis.
The prior art uses heated compressed gases such as compressed, heated air or nitrogen for a heating up large steam turbines at electrical generation stations. Moreover, when these gases are used to heat up the unit after maintenance the compression of the air or the nature of the inert gas used leaves the gas extremely dry. Nitrogen vapor typically has a dew point of −70° C. Compressed air is not as dry but typically comes out of a compressor at dew points of −10° C. or lower and is usually devoid of any water due to compression and the effect on the dew point. The lack of water in the heat up gas means that the specific heat of the heat up medium can be improved by the incorporation of a controlled amount of water vapor, which is fully absorbed into the heat up medium.
Therefore, a benefit exists to take advantage of the specific heat of water vapor into the heat up process of units that can benefit from controlled heating.